Method and device for converting  thermal energy into mechanical energy

ABSTRACT

When converting thermal into mechanical energy by a working medium containing a mixture of at least two materials having different boiling and condensation points, which is fed to a condenser, and is condensed therein, the condenser condensation pressure may increase and the efficiency for generating the mechanical energy thus decreases because the mixture of materials is separated into a liquid phase and a vapor phase upstream of the condenser. To prevent this, the liquid phase of the working medium is mixed with the vapor phase of the working medium before or while the working medium is condensed, thus once again creating a homogeneous mixture of materials which condenses at a lower pressure than the separated working medium, thereby preventing loss of efficiency. This can be applied to the use of thermal energy from low-temperature sources such as geothermal fluids, industrial waste heat, or waste heat from internal combustion engines.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage Application of InternationalApplication No. PCT/EP2008/060921 filed Aug. 21, 2008, which designatesthe United States of America, and claims priority to German ApplicationNo. 10 2007 041 458.9 filed Aug. 31, 2007, the contents of which arehereby incorporated by reference in their entirety.

TECHNICAL FIELD

The invention relates to a method and apparatus for conversion ofthermal energy to mechanical energy.

BACKGROUND

A method such as this and an apparatus such as this are known, forexample, from WO 2005/100755 A1.

In recent years, widely differing technologies have been developed forlow-temperature heat sources with temperatures up to a maximum of 400°C., for example geothermal fluids or industrial waste heat, which allowthe heat from these sources to be converted to mechanical and/orelectrical energy with high efficiency. In addition to the Rankineprocess which uses an organic agent (Organic Rankine Cycle, ORC), theso-called Kalina cycle process, in particular, is distinguished byconsiderably higher efficiencies than the classical Rankine process.Various circuits for widely differing applications have already beendeveloped on the basis of the Kalina cycle process. Instead of usingwater, these circuits use a two-substance mixture (for example ofammonia and water) as the agent, with the different boiling andcondensation temperatures of the two substances and the non-isothermalboiling and condensation process of the mixture resulting from thisbeing exploited in order to increase the efficiency of the circuit incomparison to a Rankine circuit.

A Kalina circuit such as this normally comprises at least one pump forincreasing the pressure of the agent, a heat exchanger for producing avapor phase of the agent by heat transfer from an external heat source,for example a geothermal liquid or industrial waste heat, and anexpansion device, preferably a turbine, for expansion of the vapor phaseand conversion of its thermal energy to mechanical energy. The expandedagent is then condensed in a condenser with the aid of a coolant.

Even more components may be connected in the circuit in order to improvethe efficiency. For example—as disclosed in WO 2005/100755 A1—aseparator can be arranged in the circuit between the heat exchanger andthe expansion device, by means of which any liquid phase of the agentwhich is still present in the event of any partial vaporization of theagent in the heat exchanger can be separated from the vapor phase beforebeing supplied to the expansion device. The separated liquid phase canthen be combined with the expanded vapor phase by means of a mixingdevice which is arranged in the circuit between the expansion device andthe condenser. Further heat exchangers can be provided in order totransfer heat from the expanded agent to the agent before it is suppliedto the heat exchanger.

A Kalina circuit with an ammonia-water mixture as the agent and which isknown from EP 0756069 B1 additionally has a distillation unit, which isarranged in the circuit between the condenser and the pump, forseparation of a weak ammonia liquid from the agent flow. This weakammonia liquid is supplied to the agent that has been expanded in theturbine, before this agent is supplied to the condenser.

As a result of partial condensation of the agent, the agent may containa continuously increasing proportion of the liquid phase in a lineconnection between the expansion device and the condenser. In addition,feeding a liquid phase of the agent, which for example has beenseparated before the expansion device, into the expanded vapor phaseleads to an increase in the proportion of the liquid phase in the agentbefore it is supplied to the condenser. The increasing proportion of theliquid phase leads to “demixing” of the substance mixture and to theformation of an inhomogeneous, partially demixed two-phase flow in theline connection.

For example, if the agent comprises an ammonia-water mixture, then thisresults in an inhomogeneous, partially demixed, two-phase flow in theline connection, comprising a saturated vapor which is rich in ammoniaand a condensate with little ammonia. In consequence, the condenser ispartially flooded with condensate with little ammonia, and the ammoniavapor fills only the remaining residue of the heat exchanger. Theflooded component reduces the effectiveness of the condenser.Furthermore, the condensation pressure of the vapor which is rich inammonia and which (for example comprises 95% ammonia) is considerablyhigher than that of a homogeneous water-ammonia mixture. The higher thecondensation pressure is in the condenser, the shallower, however, isthe pressure gradient to be dissipated across the turbine. Inconsequence, the circuit generates less mechanical and/or electricalpower, with a poorer efficiency.

SUMMARY

According to various embodiments, a method can be developed so as tomake it possible to avoid such efficiency losses.

According to an embodiment, a method for conversion of thermal energy tomechanical energy using an agent which comprises a substance mixturehaving at least two substances which have different boiling andcondensation temperatures, wherein the agent which is expanded in anexpansion device is supplied as a two-phase flow with a liquid phase anda vapor phase to a condenser, in which it is condensed, may comprise thestep of mixing the liquid phase with the vapor phase in the two-phaseflow before or during the condensation of the agent in the condenser.

According to a further embodiment, for mixing in the two-phase flow, theliquid phase can be separated from the vapor phase, and the separatedliquid phase is then combined with the vapor phase again, wherein theseparated liquid phase is preferably sprayed into the vapor phase forcombination. According to a further embodiment, before being sprayed in,the pressure of the separated liquid phase can be increased to a valuewhich is higher than the pressure of the vapor phase. According to afurther embodiment, the separation of the liquid phase from the vaporphase can be carried out immediately before the condenser. According toa further embodiment, the mixing process can be carried out immediatelybefore or in the condenser. According to a further embodiment, the agentmay pass through at least the following method steps in a closed circuitafter the condensation: —increasing the pressure of the agent,—producing a vapor phase of the agent by heat transfer from an externalheat source, and—expanding the vapor phase and converting its thermalenergy to mechanical energy. According to a further embodiment, beforethe expansion of the vapor phase of the agent, a liquid phase of theagent can be separated from the vapor phase, and the vapor phase can besupplied again after it has been expanded. According to a furtherembodiment, a geothermal fluid, industrial waste heat or waste heat froman internal combustion engine can be used as the external heat source.According to a further embodiment, a mixture of ammonia and water can beused as the agent.

According to another embodiment, an apparatus for conversion of thermalenergy to mechanical energy using an agent which comprises a substancemixture with at least two substances which have different boiling andcondensation temperatures, having a condenser for condensation of theagent, wherein the agent, which is expanded in an expansion device, isin the form of a two-phase flow with a liquid phase and a vapor phasebefore it is supplied to the condenser, may comprise a mixing device formixing the liquid phase of the two-phase flow with the vapor phase ofthe two-phase flow before or during the condensation of the agent in thecondenser.

According to a further embodiment, the mixing device may have aseparator for separation of the liquid phase from the vapor phase, andhas at least one nozzle for spraying the separated liquid phase into thevapor phase. According to a further embodiment, the mixing device mayhave a pump, by means of which the pressure of the separated liquidphase can be increased to a value which is higher than the pressure ofthe vapor phase. According to a further embodiment, the separator can bearranged immediately before the condenser in the flow direction of theagent. According to a further embodiment, the at least one nozzle can bearranged immediately before or in the condenser in the flow direction ofthe agent. According to a further embodiment, the agent can be carriedin a closed circuit in the apparatus, which closed circuit has at leastthe following components after the condenser in the flow direction ofthe agent: —a pump for increasing the pressure of the agent; —a heatexchanger for producing a vapor phase of the agent by heat transfer froman external heat source, and—an expansion device, in particular aturbine, for expansion of the vapor phase and conversion of its thermalenergy to mechanical energy. According to a further embodiment, thecircuit additionally may comprise a separator, which is arranged betweenthe heat exchanger and the expansion device, for separation of a liquidphase of the agent from a vapor phase, and a combination means, which isarranged between the expansion device and the mixing device, forcombination of the separated liquid phase and the expanded vapor phase.According to a further embodiment, the external heat source can be ageothermal flow, industrial waste heat or waste heat from an internalcombustion engine. According to a further embodiment, the agent can be amixture of ammonia and water.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention as well as further refinements will be explained in moredetail in the following text with reference to exemplary embodiments inthe figures, in which:

FIG. 1 shows a circuit according to one particularly embodiment,

FIG. 2 shows one example of demixing of a two-substance mixture in aline connection,

FIG. 3 shows a mixing device with spraying in jointly for a plurality ofcondensers,

FIG. 4 shows a mixing device with spraying directly into the condensers,and

FIG. 5 shows a mixing device with separate spraying in for eachindividual condenser.

DETAILED DESCRIPTION

The method according to various embodiments for conversion of thermalenergy to mechanical energy using an agent which comprises a substancemixture having at least two substances which have different boiling andcondensation temperatures, wherein the agent which is expanded in anexpansion device is supplied as a two-phase flow with a liquid phase anda vapor phase to a condenser, in which it is condensed, provides thatthe liquid phase is mixed with the vapor phase in the two-phase flowbefore or during the condensation of the agent in the condenser.

This makes it possible to avoid demixing of the two-substance mixture,allowing a homogeneous two-substance mixture to be produced again in thetwo-phase flow. If the coolant average temperature in the condenserremains constant, a homogeneous two-substance mixture actually condensesat a lower pressure. However a lower condensation pressure in thecondenser results in an increase in the pressure gradient to bedissipated across the turbine, as a result of which more mechanicaland/or electrical power can be produced, at a higher efficiency.

The liquid phase can be mixed with the vapor phase very easily byseparating the liquid phase from the vapor phase in the two-phase flowand then combining the separated liquid phase with the vapor phaseagain. The separated liquid phase is in this case preferably sprayedinto the vapor phase.

Particularly good mixing of the liquid and the vapor phases can in thiscase be achieved by increasing the pressure of the separated liquidphase to a value which is higher than the pressure of the vapor phase,in order to spray it in. The separated liquid phase is thereforesupplied to the vapor phase at an increased pressure.

In this case, separation of the liquid phase from the vapor phase ispreferably carried out immediately before the condenser, in order toavoid the two-substance mixture demixing again on its way to thecondenser.

The mixing process itself can likewise be carried out immediately beforethe condenser, or else directly in the condenser.

In this case, the agent advantageously passes through at least thefollowing method steps in a closed circuit after the condensation:

-   -   increasing the pressure of the agent,    -   producing a vapor phase of the agent by heat transfer from an        external heat source, and    -   expanding the vapor phase and converting its thermal energy to        mechanical energy.

The agent can in this case be vaporized completely by the heat transfer(that is to say only a vapor phase exists), or can be only partiallyvaporized (that is to say a vapor phase and a liquid phase exist). Inthe case of only partial vaporization, before the expansion of the vaporphase, the liquid phase of the agent is advantageously separated fromthe vapor phase, and the vapor phase is supplied again after it has beenexpanded. The liquid phase therefore bypasses an expansion device forexpansion of the vapor phase.

After expansion, the agent can be supplied to the condenser directly orvia one or more intermediate heat exchangers, which transfer the heatfrom the expanded vapor phase to the agent before its at least partialvaporization.

A geothermal fluid, industrial waste heat or waste heat from an internalcombustion engine is preferably used as the external heat source.

In this case, particularly high efficiencies can be achieved if amixture of ammonia and water is used as the agent. The apparatusaccording to various embodiments for conversion of thermal energy tomechanical energy using an agent which comprises a substance mixturewith at least two substances which have different boiling andcondensation temperatures, comprises a condenser for condensation of theagent, wherein the agent, which is expanded in an expansion device, isin the form of a two-phase flow with a liquid phase and a vapor phasebefore it is supplied to the condenser, and a mixing device for mixingthe liquid phase of the two-phase flow with the vapor phase of thetwo-phase flow before or during the condensation of the agent in thecondenser.

The mixing device advantageously has a separator for separation of theliquid phase from the vapor phase, and advantageously has at least onenozzle for spraying the separated liquid phase into the vapor phase.

If the mixing device has a pump, by means of which the pressure of theseparated liquid phase can be increased to a value which is higher thanthe pressure of the vapor phase, particularly good mixing of the twophases can be achieved when it is sprayed in.

If the separator is arranged immediately before the condenser in theflow direction of the agent, it is possible to avoid the two-substancemixture demixing again on its way to the condenser.

The at least one nozzle may itself likewise be arranged immediatelybefore or else in the condenser in the flow direction of the agent.

According to one embodiment, the agent can be carried in a closedcircuit in the apparatus, which closed circuit has at least thefollowing components after the condenser in the flow direction of theagent:

-   -   a pump for increasing the pressure of the agent    -   a heat exchanger for producing a vapor phase of the agent by        heat transfer from an external heat source, and    -   an expansion device, in particular a turbine, for expansion of        the vapor phase and conversion of its thermal energy to        mechanical energy.        In this case, the agent may be completely vaporized by the heat        transfer (that is to say only a vapor phase exists) or only        partially vaporized (that is to say a vapor phase and a liquid        phase exist). In the case of only partial vaporization, the        circuit advantageously also comprises a separator, which is        arranged between the heat exchanger and the expansion device,        for separation of a liquid phase from the vapor phase, and a        combination means, which is arranged between the expansion        device and the mixing device, for combination of the separated        liquid phase and the expanded vapor phase. In this case, the        liquid phase can in this way bypass the expansion device. The        heat source is preferably a geothermal fluid, industrial waste        heat or waste heat from an internal combustion engine. The agent        is advantageously a mixture of ammonia and water.

An apparatus 1 as shown in FIG. 1 for conversion of thermal energy tomechanical energy comprises a circuit 2 in which a pump 3 for increasingthe pressure of the agent, a heat exchanger 4 for producing a vaporphase of the agent by heat transfer from an external heat source 5, aturbine 6 for expansion of the vapor phase of the agent and conversionof its thermal energy to mechanical energy, a mixing device 7 for mixinga liquid and a vapor phase of the agent and a condenser for completecondensation of the agent with the aid of a coolant 9 are arrangedsuccessively as major components in the flow direction of an agent. Byway of example, the external heat source 5 is a geothermal fluid,industrial waste heat or waste heat from an internal combustion engine.By way of example, the turbine 6 drives a generator, which is notillustrated but converts the mechanical energy to electrical energy.

The agent comprises a substance mixture having at least two substanceswhich have different boiling and condensation temperatures. Thefollowing text is based on the assumption that a mixture of ammonia andwater is used as the agent.

As further components, the circuit 2 comprises a separator 15, which isarranged between the heat exchanger 4 and the turbine 6, for separationof a liquid phase of the agent from the vapor phase, and a combinationmeans 16, which is arranged between the turbine 6 and the mixing device7, for combination of the separated liquid phase and the expanded vaporphase.

During operation of the circuit 2, the agent is exclusively in the formof a liquid after the condenser 8. The liquid agent is raised to ahigher pressure by means of the pump 3 and is then at least partiallyvaporized in the heat exchanger 4, that is to say the agent exists in avapor phase and possibly a liquid phase with little ammonium after theheat exchanger. The liquid phase which may possibly still be present isseparated from the vapor phase in the separator 15.

The vapor phase is expanded in the turbine 6, and its thermal energy isconverted to mechanical energy. The mechanical energy can then be usedfurther, for example for electricity generation.

The vapor phase, which has now been expanded, is combined again with theliquid phase, which was possibly previously separated, in thecombination means 16.

Because of partial condensation of the expanded vapor phase and possiblyliquid phase supplied via the combination means 16 the proportion ofliquid in the ammonium-water mixture will increase in the lineconnection 10 between the turbine 6 and the condenser 8, with demixingtaking place into saturated vapor 11 which is rich in ammonia, andcondensate 12 with little ammonia (see FIG. 2). The condenser 8 wouldtherefore be supplied with an inhomogeneous, partially demixed agentflow. This would result in the condenser 8 being partially flooded withthe condensate 12 with little ammonia, with the saturated vapor 11 whichis rich in ammonia filling the rest of the condenser. The floodedcomponent would decrease the effectiveness of the condenser and wouldtherefore increase the condensation pressure, since the condensationpressure of the saturated vapor which is rich in ammonia (approximately95% ammonia) is considerably higher than that of a homogeneouswater-ammonia mixture. As the condensation pressure rises in thecondenser, however, the pressure gradient to be dissipated across theturbine decreases, and therefore the mechanical and/or electrical powerwhich can be produced also decreases.

In order to avoid such efficiency losses, the circuit 2 has a mixingdevice 7. The mixing device 7 comprises a separator 20 for separation ofthe liquid phase with little ammonia from the vapor phase which is richin ammonia, and a nozzle 21 for spraying the separated liquid phase intothe vapor phase, wherein the separator 20 and the nozzle 21 are arrangedsuccessively in the connecting line 10, between the turbine 6 and thecondenser 8 and after the combination means 16, in the flow direction ofthe agent. The liquid phase which is separated in the separator 20 issupplied via a bypass line 14 to the nozzle 21. A pump 22 and a controlvalve 23 are connected in the bypass line 14.

The pump 22 makes it possible to increase the pressure on the separatedliquid phase which carried in the bypass line 14 to a value which ishigher than the pressure of the vapor phase after the separator 20. Theamount of liquid phase supply to the nozzle 21 can be controlled bymeans of the control valve 23.

The separator 20 is arranged immediately before the condenser 8 in theflow direction of the agent, in order to avoid demixing of the agentagain on the rest of its way to the condenser 8. The nozzle 21 can bearranged immediately before or in the condenser 8, in the flow directionof the agent.

The separator 20 therefore separates the vapor phase which is rich inammonia, from the liquid phase, with little ammonia.

The liquid phase, with little ammonia is passed to the nozzle 21 via thebypass line 14. In this case, the pump 22 increases the pressure of theliquid phase with little ammonia to a value which is higher than thepressure of the vapor phase which is rich in ammonia. The liquid phasewith little ammonia is thus sprayed at an increased pressure into thevapor phase, which is rich in ammonia in the nozzle 21. This once againresults in a homogeneous ammonia-water mixture being able to be producedand being able to be supplied to the condenser 8, which mixture actuallycondenses at a lower pressure than the vapor phase, which is rich inammonia, assuming that the cooling temperature in the condenser remainsconstant. However, with a lower condensation pressure in the condenser,the pressure gradient to be dissipated across the turbine rises, and thecircuit can therefore produce more electrical power, at a higherefficiency.

When there are a plurality of condensers 8 connected in parallel in theflow direction of the agent—as illustrated in FIG. 3—a mixing device 7can be provided with a single separator 20 and a single nozzle 21 forall the condensers 8. The separator 20 and the nozzle 21 are thenpreferably arranged immediately before the condensers 8. The liquidphase is therefore sprayed jointly into the vapor phase for all thecondensers 8.

Alternatively, when there are a plurality of condensers 8 which areconnected in parallel in the flow direction of the agent, it is alsopossible to provide a mixing device 7 with a single separator 20 and ineach case one or more nozzles 21 for each of the condensers 8. In theexemplary embodiment shown in FIG. 4, the separator 20 is arrangedimmediately in front of the condensers 8, and the nozzles 21 arearranged in the condensers 8. The liquid phase is therefore sprayeddirectly into the condensers 8. In this case, the supply of the liquidphase to the nozzles 21 can be controlled by means of a joint controlvalve 23.

However, as illustrated in FIG. 5, the nozzles 21 can also be arrangedimmediately before the respective condensers 8, that is to say thespraying-in process is carried out separately for each individualcondenser 8. In this case, supply of the liquid phase to each of thenozzles 21 can be controlled by means of a separate control valve 23 foreach of the condensers 8.

1. A method for conversion of thermal energy to mechanical energy usingan agent which comprises a substance mixture having at least twosubstances which have different boiling and condensation temperatures,the method comprising the steps of: supplying the agent which isexpanded in an expansion device as a two-phase flow with a liquid phaseand a vapor phase to a condenser, in which it is condensed, and mixingthe liquid phase with the vapor phase in the two-phase flow before orduring the condensation of the agent in the condenser.
 2. The methodaccording to claim 1, wherein for mixing in the two-phase flow, theliquid phase is separated from the vapor phase, and the separated liquidphase is then combined with the vapor phase again, wherein the separatedliquid phase is preferably sprayed into the vapor phase for combination.3. The method according to claim 2, wherein before being sprayed in, thepressure of the separated liquid phase is increased to a value which ishigher than the pressure of the vapor phase.
 4. The method according toclaim 2, wherein the separation of the liquid phase from the vapor phaseis carried out immediately before the condenser.
 5. The method accordingto claim 1, wherein the mixing process is carried out immediately beforeor in the condenser.
 6. The method according to claim 1, wherein theagent passes through at least the following method steps in a closedcircuit after the condensation: increasing the pressure of the agent,producing a vapor phase of the agent by heat transfer from an externalheat source, and expanding the vapor phase and converting its thermalenergy to mechanical energy.
 7. The method according to claim 6, whereinbefore the expansion of the vapor phase of the agent, a liquid phase ofthe agent is separated from the vapor phase, and the vapor phase issupplied again after it has been expanded.
 8. The method according toclaim 6, wherein a geothermal fluid, industrial waste heat or waste heatfrom an internal combustion engine is used as the external heat source.9. The method according to claim 1, wherein a mixture of ammonia andwater is used as the agent.
 10. An apparatus for conversion of thermalenergy to mechanical energy using an agent which comprises a substancemixture with at least two substances which have different boiling andcondensation temperatures, having a condenser for condensation of theagent, wherein the agent, which is expanded in an expansion device, isin the form of a two-phase flow with a liquid phase and a vapor phasebefore it is supplied to the condenser, the apparatus comprising amixing device for mixing the liquid phase of the two-phase flow with thevapor phase of the two-phase flow before or during the condensation ofthe agent in the condenser.
 11. The apparatus according to claim 10,wherein the mixing device has a separator for separation of the liquidphase from the vapor phase, and has at least one nozzle for spraying theseparated liquid phase into the vapor phase.
 12. The apparatus accordingto claim 11, wherein the mixing device has a pump, by means of which thepressure of the separated liquid phase can be increased to a value whichis higher than the pressure of the vapor phase.
 13. The apparatusaccording to claim 11, wherein the separator is arranged immediatelybefore the condenser in the flow direction of the agent.
 14. Theapparatus according to claim 11, wherein the at least one nozzle isarranged immediately before or in the condenser in the flow direction ofthe agent.
 15. The apparatus according to claim 10, wherein the agentcan be carried in a closed circuit in the apparatus, which closedcircuit has at least the following components after the condenser in theflow direction of the agent: a pump for increasing the pressure of theagent a heat exchanger for producing a vapor phase of the agent by heattransfer from an external heat source, and an expansion device forexpansion of the vapor phase and conversion of its thermal energy tomechanical energy.
 16. The apparatus according to claim 15, wherein thecircuit additionally comprises a separator, which is arranged betweenthe heat exchanger and the expansion device, for separation of a liquidphase of the agent from a vapor phase, and a combination means, which isarranged between the expansion device and the mixing device, forcombination of the separated liquid phase and the expanded vapor phase.17. The apparatus according to claim 15, wherein the external heatsource is a geothermal flow, industrial waste heat or waste heat from aninternal combustion engine.
 18. The apparatus according to claim 10,wherein the agent is a mixture of ammonia and water.
 19. The apparatusaccording to claim 15, wherein the expansion device is a turbine.
 20. Amethod for conversion of thermal energy to mechanical energy, comprisingthe steps of: providing an agent which comprises a substance mixturehaving at least two substances which have different boiling andcondensation temperatures, supplying the agent which is expanded in anexpansion device as a two-phase flow with a liquid phase and a vaporphase to a condenser, in which it is condensed, and mixing the liquidphase with the vapor phase in the two-phase flow before or during thecondensation of the agent in the condenser.